Glenayre Document Number: 9110.01002
GL-T8541 PA
09/10/99
THEORY OF OPERATION
t8541
p6.
fdk
Copyright © 1997 Glenayre
Page: 6-5
Figure 6-3 PA Board Block Diagram
6.4.1 PA Bias and Idle Current Adjustment
The PA board bias is determined by the AGC voltage, which is applied to the gate of the
two RF amplifier transistors through the bias adjustment R7. Adjustment of R7 is made by
observing the PA board idle current on the DSP exciter VDT screen. When this adjustment
is being performed, the transmitter must be keyed with no RF power delivered from the
DSP exciter or driver and the AGC voltage set at maximum. This condition is achieved by
placing the transmitter in FWD PWR CAL mode and keying the transmitter with no RF
input applied (exciter RF output cable disconnected.) Refer to the VDT manual for addi-
tional information regarding the exciter VDT display. The correct idle current adjustment
under these conditions is 2.0 A for each PA board.
The idle current adjustment has two notable effects on the PA parameters, they are:
1.
The PA gain is changed by changing the amount of input drive needed.
2.
When the transmitter is keyed, some PA current will always flow. If the PA current
value is low or zero when the PA is keyed, it is an indication that one or both of the RF
transistors on the PA board are defective. This will cause the DSP exciter to report an
alarm or fault condition.
CR1
CR2
5.1V
Q1
Q2
R7
R8, R9
R3
R4
Ferrite
Beads
L7, Coax
Transformer
L8, Coax
Transformer
RF Input
From 1 to 3
Splitter
RF Output
to 3 to 1
Combiner
AGC Voltage
From Metering
Board
+28 Vdc
From Metering
Board
